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Cara Memilih Material Pipa

"PIPA SEAMLESS" Material pipa ada berbagai jenis. Bagaimana cara untuk menentukan material yang harus digunakan? Untuk dapat menentukan material, terutama untuk bidang industri, faktor yang paling penting adalah fluida apa yang akan mengalir didalamnya. Selain itu, kondisi luar dari pipa juga akan mempengaruhi. Dan terakhir, tentu saja dalam sisi ekonomi juga akan menjadi dasar pemilihan material.
Pipa juga dapat dibagi menjadi 2 bagian besar. Pipa dari logam dan non-logam. Logam telah terdiri dari carbon steel, stainless steel, aluminium, nickel dan lainnya. Berikut ini adalah contoh dalam desain pipa untuk pabrik industri gas alam, minyak, atau pabrik kimia lainnya.
Pertama, insinyur proses harus bisa menghitung apa dan berapa banyak macam kandungan yang akan melewati pipa. Pada dasarnya, semua pipa untuk proses biasanya harus memakai pipa logam dan dimulai dari material carbon steel yang paling murah.
Akibat aliran fluida, bagian dalam pipa akan mengalami korosi, dan salah satu cara untuk dapat menetapkan kecepatan korosi adalah memakai grafik de Waard – Milliams nomograph. Grafik ini membantu untuk dapat menentukan berapa kecepatan korosi (mm/tahun) yang telah disebabkan adanya kandungan CO2 dalam fluida.
Problem disebabkan korosi juga dapat diatasi dengan menambah ketebalan pipa sebesar kecepatan korosi dikali tahun lamanya pabrik didesain. Tetapi, jika total ketebalan yang dibutuhkan untuk dapat mengatasi korosi itu terlalu tebal, pipa akan menjadi sangat tebal dan tidak efektif dalam pembangunannya. Untuk keadaan ini, pipa dari stainless steel telah menjadi pilihan selanjutnya.
Selain korosi, suhu fluida juga dapat menentukan material pipa. Semakin rendah suhu, logam akan menjadi mudah mengalami retakan. Ini karena sifat brittle (getas)  logam bertambah pada suhu rendah . Stainless steel merupakan salah satu yang tahan akan suhu rendah. Karena itu, untuk cryogenic service (fluida dengan suhu operasi dibawah -196 degC) stainless steel adalah material yang cocok dibandingkan dengan carbon steel.
Stainless steel sering disebut juga corrosion resistance alloy (campuran logam tahan korosi) dan tentunya akan lebih mahal jika dibandingkan carbon steel. Stainless steel bisa dibagi menjadi beberapa jenis, contohnya austenitic, feritic, martenistic, duplex dan high alloy stainless steel (campuran tinggi logam stainless steel). Sayangnya, stainless steel tidak tahan terhadap semua jenis korosi, terutama korosi yang disebabkan oleh klorida, sulfida serta fluida asam (sour fluid) lainnya.
Untuk sistem pipa yang mengalirkan fluida asam (piping system for sour service) biasanya di desain berdasarkan standar NACE (National Association of Corrosion Engineers) MR0175. Mulai tahun 2003, standar NACE MR0175 bersatu dengan ISO 15156 dan yang telah memiliki syarat desain yang sulit dibandingkan edisi tahun sebelumnya.

Berdasarkan NACE MR0175/ISO 15156, penggunaan austenitic stainless steel telah dibatasi oleh kombinasi dari kadar khlorida, H2S (hydrogen sulfide) dan suhu fluida. Jika austenitic stainless steel tidak dapat digunakan, maka penggunaan duplex atau high alloy stainless steel merupakan pilihan selanjutnya. Jika duplex atau high alloy stainless steel juga tidak dapat digunakan, maka pilihan selanjutnya adalah menggunakan logam campuran nikel seperti incoloy dan inconel.
Semakin tahan terhadap berbagai korosi, semakin mahal harga material tersebut. Untuk dapat mengurangi biaya, pengaplikasian cladding atau overlay juga merupakan salah satu alternatif. Misalnya dengan menggunakan pipa dari carbon steel dengan dilapisi logam mahal pada bagian dalamnya saja yang bersentuhan langsung dengan fluida sumber korosi akan bisa menekan biaya tanpa mengurangi ketahanan terhadap korosi.
Pemilihan material ini bukan hanya untuk pipa, tetapi juga berlaku untuk bejana (vessel), katup (valve) dan elemen pipa lainnya. Untuk katup, walaupun material dari badan katup bisa memakai carbon steel, tetapi bagian dimana korosi tidak diperbolehkan untuk dapat menjaga kemampuan katup untuk menyekat (sering disebut sebagai trim, seperti bagian valve seat, stem dan lainnya), maka penggunaan stainless steel atau logam tahan korosi lainnya menjadi keharusan.
Pada saat melakukan pemilihan material yang sebenarnya, mungkin tidak akan semudah yang dijabarkan diatas, tetapi secara umum, begitulah proses pemilihan material pada saat mendesain pabrik industri.


Editor : Dian Sukmawati

CARA MEMILIH MATERIAL PIPA

Late in April, after Native American actors walked off in disgust from the set of Adam Sandler’s latest film, a western sendup that its distributor, Netflix, has defended as being equally offensive to all, a glow of pride spread through several Native American communities.

Tantoo Cardinal, a Canadian indigenous actress who played Black Shawl in “Dances With Wolves,” recalled thinking to herself, “It’s come.” Larry Sellers, who starred as Cloud Dancing in the 1990s television show “Dr. Quinn, Medicine Woman,” thought, “It’s about time.” Jesse Wente, who is Ojibwe and directs film programming at the TIFF Bell Lightbox in Toronto, found himself encouraged and surprised. There are so few film roles for indigenous actors, he said, that walking off the set of a major production showed real mettle.

But what didn’t surprise Mr. Wente was the content of the script. According to the actors who walked off the set, the film, titled “The Ridiculous Six,” included a Native American woman who passes out and is revived after white men douse her with alcohol, and another woman squatting to urinate while lighting a peace pipe. “There’s enough history at this point to have set some expectations around these sort of Hollywood depictions,” Mr. Wente said.

The walkout prompted a rhetorical “What do you expect from an Adam Sandler film?,” and a Netflix spokesman said that in the movie, blacks, Mexicans and whites were lampooned as well. But Native American actors and critics said a broader issue was at stake. While mainstream portrayals of native peoples have, Mr. Wente said, become “incrementally better” over the decades, he and others say, they remain far from accurate and reflect a lack of opportunities for Native American performers. What’s more, as Native Americans hunger for representation on screen, critics say the absence of three-dimensional portrayals has very real off-screen consequences.

“Our people are still healing from historical trauma,” said Loren Anthony, one of the actors who walked out. “Our youth are still trying to figure out who they are, where they fit in this society. Kids are killing themselves. They’re not proud of who they are.” They also don’t, he added, see themselves on prime time television or the big screen. Netflix noted while about five people walked off the “The Ridiculous Six” set, 100 or so Native American actors and extras stayed.

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But in interviews, nearly a dozen Native American actors and film industry experts said that Mr. Sandler’s humor perpetuated decades-old negative stereotypes. Mr. Anthony said such depictions helped feed the despondency many Native Americans feel, with deadly results: Native Americans have the highest suicide rate out of all the country’s ethnicities.

The on-screen problem is twofold, Mr. Anthony and others said: There’s a paucity of roles for Native Americans — according to the Screen Actors Guild in 2008 they accounted for 0.3 percent of all on-screen parts (those figures have yet to be updated), compared to about 2 percent of the general population — and Native American actors are often perceived in a narrow way.

In his Peabody Award-winning documentary “Reel Injun,” the Cree filmmaker Neil Diamond explored Hollywood depictions of Native Americans over the years, and found they fell into a few stereotypical categories: the Noble Savage, the Drunk Indian, the Mystic, the Indian Princess, the backward tribal people futilely fighting John Wayne and manifest destiny. While the 1990 film “Dances With Wolves” won praise for depicting Native Americans as fully fleshed out human beings, not all indigenous people embraced it. It was still told, critics said, from the colonialists’ point of view. In an interview, John Trudell, a Santee Sioux writer, actor (“Thunderheart”) and the former chairman of the American Indian Movement, described the film as “a story of two white people.”

“God bless ‘Dances with Wolves,’ ” Michael Horse, who played Deputy Hawk in “Twin Peaks,” said sarcastically. “Even ‘Avatar.’ Someone’s got to come save the tribal people.”

Dan Spilo, a partner at Industry Entertainment who represents Adam Beach, one of today’s most prominent Native American actors, said while typecasting dogs many minorities, it is especially intractable when it comes to Native Americans. Casting directors, he said, rarely cast them as police officers, doctors or lawyers. “There’s the belief that the Native American character should be on reservations or riding a horse,” he said.

“We don’t see ourselves,” Mr. Horse said. “We’re still an antiquated culture to them, and to the rest of the world.”

Ms. Cardinal said she was once turned down for the role of the wife of a child-abusing cop because the filmmakers felt that casting her would somehow be “too political.”

Another sore point is the long run of white actors playing American Indians, among them Burt Lancaster, Rock Hudson, Audrey Hepburn and, more recently, Johnny Depp, whose depiction of Tonto in the 2013 film “Lone Ranger,” was viewed as racist by detractors. There are, of course, exceptions. The former A&E series “Longmire,” which, as it happens, will now be on Netflix, was roundly praised for its depiction of life on a Northern Cheyenne reservation, with Lou Diamond Phillips, who is of Cherokee descent, playing a Northern Cheyenne man.

Others also point to the success of Mr. Beach, who played a Mohawk detective in “Law & Order: Special Victims Unit” and landed a starring role in the forthcoming D C Comics picture “Suicide Squad.” Mr. Beach said he had come across insulting scripts backed by people who don’t see anything wrong with them.

“I’d rather starve than do something that is offensive to my ancestral roots,” Mr. Beach said. “But I think there will always be attempts to drawn on the weakness of native people’s struggles. The savage Indian will always be the savage Indian. The white man will always be smarter and more cunning. The cavalry will always win.”

The solution, Mr. Wente, Mr. Trudell and others said, lies in getting more stories written by and starring Native Americans. But Mr. Wente noted that while independent indigenous film has blossomed in the last two decades, mainstream depictions have yet to catch up. “You have to stop expecting for Hollywood to correct it, because there seems to be no ability or desire to correct it,” Mr. Wente said.

There have been calls to boycott Netflix but, writing for Indian Country Today Media Network, which first broke news of the walk off, the filmmaker Brian Young noted that the distributor also offered a number of films by or about Native Americans.

The furor around “The Ridiculous Six” may drive more people to see it. Then one of the questions that Mr. Trudell, echoing others, had about the film will be answered: “Who the hell laughs at this stuff?”

Native American Actors Work to Overcome a Long-Documented Bias

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